Oxygenate to olefins reaction systems typically convert oxygenates to olefin products. In particular, methanol to olefins reaction systems utilize methanol as the primary feed for the conversion process, and these processes typically use molecular sieves as catalysts.
Silicoaluminophosphate (SAPO) molecular sieves have generally been considered to be desirable catalytic materials in converting oxygenate feedstocks to olefin compositions. These catalysts are particularly good catalysts for making olefins such as ethylene and propylene from oxygenate compounds.
Alternative catalysts to the SAPO molecular sieves have also been sought. Particularly desirable alternatives have included those catalysts that have a high selectivity to ethylene and propylene, are highly attrition resistant and are, of course, those that are more efficiently used in the overall oxygenate to olefin production process.
U.S. Pat. No. 7,071,136 discloses molecular sieves containing [AlO4] and [SiO4] tetrahedral units can be used as a catalyst to convert methanol to olefins such as ethylene and propylene. The particular catalysts are considered to be highly attrition resistant, which is a preferred characteristic for the operation of the reaction system.
U.S. Pat. No. 6,844,291 B2 discloses a molecular sieve catalyst composition that includes a metal oxide. Combining the metal oxide with the molecular sieve was considered to enhance olefin yield and catalyst lifetime in the oxygenate to olefin reaction process.
U.S. Patent Publ. No. 2003/0176751 describes a porous crystalline material that has a chabazite framework type and involves the molar relationship: X2O3:(n)YO2, where X is a trivalent element, such as aluminum, boron, iron, indium, and/or gallium; Y is a tetravalent element such as silicon, tin, titanium and/or germanium; and n is greater than 100. The material is synthesized in a fluoride medium and exhibits activity and selectivity in the conversion of methanol to lower olefins, especially ethylene and propylene.
U.S. Pat. No. 7,094,389 discloses a crystalline material substantially free of framework phosphorus and comprising a CHA framework type molecular sieve with stacking faults or at least one intergrown phase of a CHA framework type molecular sieve and an AEI framework type molecular sieve. The material in its calcined, anhydrous form, involves the molar relationship: (n)X2O3:YO2, where X is a trivalent element; Y is a tetravalent element; and n is from 0 to about 0.5. The material exhibits activity and selectivity in the conversion of methanol to lower olefins, especially ethylene and propylene.
Various catalyst pretreatment methods are also used to increase the amount of light or prime olefins (i.e., ethylene or propylene, or mixtures thereof) produced in the methanol to olefins conversion processes. For example, U.S. Pat. No. 7,045,672 is directed to processes for making olefin product from an oxygenate feed that includes a step of pretreating a fresh or regenerated metalloaluminophosphate molecular sieve, which is low in carbon content, with a dimethyl ether composition. The dimethyl ether in the composition forms a hydrocarbon co-catalyst within the pore structure of the molecular sieve, and the pretreated molecular sieve containing the co-catalyst is used to convert oxygenate to an olefin product, with high selectivity to light olefins.
Although advances have been made in increasing the amount of ethylene and propylene produced in the oxygenate to olefins conversion reaction, further increases in these amounts are sought. In certain cases, it is particularly desirable to increase the amount of ethylene produced relative to the propylene.